T24a: Agent, tool calls

- ToolCallCard: a step mark, the tool in mono, what it touched and how long
  it took; a press opens its arguments and result as code, an error in red
- ToolCallGroup folds several calls under one line
- ToolApprovalDialog over overlays::Dialog: Deny or Run, one answer; Escape,
  the scrim and the close deny
- PermissionPrompt: deny, always allow or allow once, in place
- chat::StepState gains Failed; chat::step_mark draws a step's mark for
  search steps and tool calls alike
- typography::format::took reads ms, seconds or minutes
- gallery: an Agent page with calls in each state, a group, the dialog and
  the prompt

AGENTS.md

@@ -133,6 +133,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - gpui 0.2.2 never clears `debug_bounds`: a test sees an element appear, not leave.
 - The system time zone comes from `typography::format::system_zone`, which fails loud; jiff's `TimeZone::system()` falls back to UTC in silence.
 - A list opens with its cursor on the current choice: Select on the chosen row, Combobox on its value, MultiSelect on the first one ticked.
+- A search step, a tool call or an agent's step says where it stands with `chat::StepState` and draws it with `chat::step_mark`.
 
 ## Decisions